Did you try gl_FragColor = vec4(1.0);? This will show you the geometry only (everything is drawn white). If it's still black, your geometry / vertex shader is wrong; if it's not, your colors are wrong. Also, make sure that your shader program is compiled and linked correctly (It's always a good idea to dump the GLSL compiler message)
–
leemesJan 19 '13 at 19:57